THIS could be the start of something big. In the next few weeks, the UK government is expected to sign a deal to start building the country's first new nuclear reactors for 25 years.

The reactor design is more efficient, and safer, than those in existing plants. They are also good news in terms of climate change (see "Shooting at climate targets"), but what they will do to energy prices is less clear. If the plan is to improve the UK's green credentials and energy security, however, yet more reactors are needed.

Assuming the deal goes ahead, the two reactors, which could cost about £14 billion, will be built at Hinkley Point on the Somerset coast, which already has two nuclear power stations, one of which is now being decommissioned. Once up and running, the two reactors will provide about 7 per cent of the UK's electricity. They will be built by a consortium led by EDF Energy and will be half-owned by China.

The reactors will be European pressurised reactors (EPRs), developed by French energy giant Areva. They are a variant on the pressurised water reactor, so named because the cooling water surrounding the reactor core is kept at high pressure to prevent it from boiling (see diagram). The design features crucial advances. It generates more electricity from less fuel and needs less downtime for maintenance. It also cuts the risk of major accidents. "It is unquestionably a safer system," says Timothy Abram of the University of Manchester in the UK.

The reactor building will be more resistant to aircraft strikes. Before 9/11, reactors were only built to survive the impact of a crashing fighter jet. "It never occurred to us that anyone would do this deliberately," says Abram. Modern aircraft can have engines as big as buses, which could punch through concrete. So the Hinkley reactors will have two concrete walls, each more than 1 metre thick.

Lessons have also been learned from the 2011 Fukushima disaster. There, the reactors shut down safely after an earthquake, but a tsunami then knocked out the back-up diesel generators. Without power, cooling systems failed and the reactors overheated. To prevent this, Hinkley will have six generators instead of four, in widely spaced waterproof buildings.

In the worst case, the EPR should contain a meltdown. As in previous designs, the reactor will be built on a 6-metre-thick concrete base to stop the molten core burning through the floor. But in the EPR, channels carved into the concrete will allow the mess to spread out. Combined with underfloor cooling, this should quickly bring down the temperature of a molten core.

And Hinkley could still arrive late. Being a new design, EPRs have been delayed and over budget. The world's first EPR at Olkiluoto in Finland is still not finished after eight years. Construction started on an EPR at Flamanville in France in 2007, but completion has been delayed until 2016. However, two EPRs at Taishan, China, should be finished within a year. Hopefully, EDF and Areva will have learned from these experiences.

In the meantime, the sticking point is the price of Hinkley's electricity. EDF wants a "strike price", guaranteeing them a minimum fee. That could inflate UK energy prices, which are already rising sharply. But to improve energy security and cut emissions, it may be a price worth paying.

"It's probably the only way you're going to get a nuclear power plant built by a private company," says Jim Watson of the UK Energy Research Centre in London. He says the UK's energy prices are mainly driven by the cost of gas, and avoiding pricey options like nuclear and renewables might not save much.

Last year the UK used 2400 terawatt hours (TWh) of energy. Electricity constituted 317 TWh, including 70 TWh of nuclear. Hinkley will add 24 TWh, which would meet a big chunk of electricity demand, but only partly replace the old reactors about to close.

To keep on track for 2050, the UK's Committee on Climate Change wants the electricity grid carbon-free by 2030. That means building lots of clean energy – not all nuclear, but equivalent to 11 Hinkleys – within 17 years, says Kevin Anderson of the University of Manchester, UK. By 2050, energy use should fall to around 1400 TWh, thanks to better energy efficiency. But the UK will increasingly use electricity for heating and transport, so electricity use may rise to 500 TWh. That is a lot, and nuclear power may need to supply 200 TWh. That means 10 more plants like Hinkley.

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A third nuclear power station at Hinkley Point will have two reactors (Image: Martin Bond/SPL)